The present invention relates to cathode-ray-tube circuits in general, and in particular to a dynamic focus signal shaper circuit for a cathode-ray-tube having an electrostatic or electromagnetic focusing lens.
The focusing of an electron beam by electric or magnetic fields in a cathode-ray-tube, hereinafter abbreviated CRT, is closely analogous to the focusing of light rays in an optical system. Not surprisingly, the electro-optical system of a CRT is plagued by distortions very similar to those which plague optical systems.
One of the more troublesome distortions is defocusing. As an electron beam leaves the cathode of a CRT, it is comparatively broad. A focusing lens, situated between the cathode and the CRT system screen, is adjusted to cause the beam to converge to a tight spot as it reaches the screen. The lens adjustment depends on the distance the beam must travel to the screen. In most CRT's the distance a beam must travel varies with the distance of deflection of the beam from screen center. Thus if a focusing lens is adjusted to provide good focus at screen center, the beam will be defocused when deflected from center.
To solve this problem focusing lenses, driven by a focusing signal from a dynamic focus signal shaper, are designed to dynamically change focal length. The dynamic focus signal shaper typically derives the focusing signal from signals proportional to beam deflection. The required focusing signal is approximately proportional to the square of the distance of deflection from screen center and is thus greatest at the corners of a rectangular CRT. Due to manufacturing tolerances, the effective focal length deviates from the square of the distance from screen center in a non-symmetrical fashion.
To accommodate these non-symmetrical variations in focal length for beam deflections in the +X, +Y, -X and -Y directions from center, previous schemes for dynamic focus signal shapers have provided for independent focus adjustment of each edge of a rectangular CRT display. Corner focus is then extrapolated from center and adjacent edge values. If the resulting corner focus is not adequate, further improvement in corner focus requires interactive compromising of adjacent edge focus. Under this scheme, optimization of corner focus is often difficult.
What is needed then, is a dynamic focus signal shaper for a rectangular CRT for producing a focusing signal proportional to the square of the distance of deflection of the beam from display center and permitting independent focus adjustment of each corner of the display. Edge focus is then interpolated from adjacent corner focus. Previously issued U.S. Pat. No. 4,429,112 discloses a related concept for astigmatism correction but does not disclose an implementing circuit. The article "Correction of Deflection-Abberations by Analog Computer" by K. Schlesinger and R. A. Wagner in Vol. 112, No. 8 IEEE Transactions on Electron Devices, herein incorporated by reference, describes a system for dynamic control of focus using a complex analog computer to generate the necessary signals to correct the display. The circuits of Schlesinger and Wagner, however, contain no provision for independent adjustment of corner focus.